1. Field of the Invention
The present invention relates generally to semiconductor devices, and more specifically, to trench type power semiconductor devices and a method for fabricating the same.
2. Description of Related Art
Trench type power semiconductor devices such as power MOSFETs are well known. Referring to FIG. 1, an example of a power MOSFET 100 according to prior includes a plurality of trenches 12 formed in semiconductor body 14. Semiconductor body 14 is usually a silicon die that includes an epitaxially grown silicon layer (epitaxial silicon layer) 16 of one conductivity (e.g. N-type) formed over a silicon substrate 18 of the same conductivity, but of higher concentration of impurities. A channel region 20 (sometimes referred to as body region) is formed in epitaxial silicon layer 16 and extends from the top surface of the semiconductor body to a first depth. Channel region 20 has a conductivity opposite to that of epitaxial layer 16 (e.g. P-type). Formed within channel region 20 are source regions 22, which have the same conductivity (e.g. N-type) as epitaxial silicon layer 16.
As is well known, trenches 12 typically extend to a depth below the bottom of channel region 20 and include gate insulation 24, which may be formed with silicon dioxide, on at least the sidewalls of trenches 12. The bottom of each trench 12 is also insulated with silicon dioxide or the like. Gate electrodes 26 are disposed within each trench 12 and again, typically extend to a depth below the depth of channel region 20. Gate electrodes 26 are typically composed of conductive polysilicon.
A typical trench type power MOSFET further includes a source electrode 28, which is electrically connected to source regions 22, and high conductivity contact regions 30, which are also formed in channel region 20. High conductivity contact regions 30 are highly doped with dopants of the same conductivity as channel region 20 (e.g. P-type) in order to reduce the contact resistance between source electrode 28 and channel region 20. A typical trench type power MOSFET 10 further includes a drain electrode 32 in electrical contact with silicon substrate 18.
As is well known, the density of the current that power MOSFET 100 may accommodate is directly proportional to the cell density of the device. Thus, the greater the number of trenches per unit area the more current the device can handle. Because of this relationship, it is desirable to pack as many trenches as possible for a given die area. One way to accomplish this is by reducing the trench pitch, which, for example, requires reducing the width of source regions 22 and/or high conductivity contact regions 30. However, traditional fabrication processes can limit the amount of reduction that can be achieved in these dimensions, thereby affecting the amount of reduction that can be achieved in trench pitch.
As is also known, in prior art power semiconductor devices, such as MOSFET 100, trenches 12 must extend at least through the entire thickness of the channel region 20. Furthermore, the gate electrode 26 must also extend at least the length of the region that is to be inverted within the channel region. Naturally, when the thickness of the channel region is increased (e.g., to increase the breakdown voltage of the device) the gate trenches must be deeper and consequently the gate electrodes larger. Having larger gate electrodes is undesirable, however, as they include a larger volume of conductive material requiring a higher amount of charge to operate. Furthermore, a thicker channel region increases the on state resistance of the device as it increases the current path.